SONOS stack with split nitride memory layer

ABSTRACT

A semiconductor device includes a substrate, a first oxide layer formed on the substrate, an oxygen-rich nitride layer formed on the first oxide layer, a second oxide layer formed on the oxygen-rich nitride layer, and an oxygen-poor nitride layer formed on the second oxide layer.

PRIORITY CLAIM

This application claims priority under 35 U.S.C. 119 to U.S. provisionalapplication No. 61/172,320, filed on Apr. 24, 2009, which isincorporated herein by reference in its entirety.

BACKGROUND

Non-volatile semiconductor memories, such as a split gate flash memory,sometimes use a stacked floating gate structure, in which electrons areinduced into a floating gate of a memory cell to be programmed bybiasing a control gate and grounding a body region of a substrate onwhich the memory cell is formed.

An oxide-nitride-oxide (ONO) stack may be used as either a chargestoring layer, as in silicon-oxide-nitride-oxide-silicon (SONOS)transistor, or as an isolation layer between the floating gate andcontrol gate, as in a split gate flash memory.

FIG. 1 is a partial cross-sectional view of a structure for asemiconductor device 100 having a SONOS gate stack or structure 102. Thestructure 100 includes a conventional ONO stack 104 formed over asurface 106 of a silicon substrate 108. The device 100 typically furtherincludes one or more diffusion regions 110, such as source and drainregions, aligned to the gate stack and separated by a channel region112. The SONOS structure 102 includes a polysilicon gate layer 114formed upon and in contact with the ONO stack 104. The poly gate 114 isseparated or electrically isolated from the substrate 108 by the ONOstack 104. The ONO stack 104 generally includes a lower (tunnel) oxidelayer 116, a nitride or oxynitride layer 118 which serves as a chargestoring or memory layer for the device 100, and a top oxide layer 120overlying the nitride or oxynitride layer 118.

One problem with this conventional SONOS structure 102 is the poor dataretention of the nitride or oxynitride layer 118 that limits the device100 lifetime and/or its use in several applications due to leakagecurrent through the layer. Another problem with conventional SONOSstructures 102 is that the stochiometry of the layer 118 is nonuniformacross the thickness of the layer. In particular, the layer 118 isconventionally formed or deposited in a single step using a singleprocess gas mixture and fixed or constant processing conditions in anattempt to provide a homogeneous layer having a high nitrogen and highoxygen concentration across the thickness of the relatively thick layer.However, this may result in nitrogen, oxygen and silicon concentrationsthat vary throughout the conventional layer 118. Consequently, thecharge storage characteristics, and in particular programming and erasespeed and data retention of a memory device 100 made with the ONO stack104, are adversely effected.

FIGS. 2-5 illustrate charge retention and migration in a conventionalSONOS structure such as the one illustrated in FIG. 1. Charge traps aredistributed through the nitride layer 118. The distribution of traps isuniform under ideal stochiometric conditions (FIG. 2), but typically thedistribution would not be so ideally uniform. When an ERASE (FIG. 3) isperformed, holes migrate toward the blocking oxide 120. Electron chargeaccumulates at the layer boundaries after programming (FIG. 4). Thisstored charge distribution can lead to significant leakage due totunneling at the nitride boundaries, for example by the processillustrated in the energy diagram FIG. 5, in which stored chargetransitions among trapped states (e.g. E_(TA), E_(TD)) to cause leakage.

Thus there is an ongoing need for a memory device that exhibits improveddata retention and improved stochiometry.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a conventional SONOS structure.

FIGS. 2-4 illustrate charge retention and migration in a conventionalSONOS structure such as the one illustrated in FIG. 1.

FIG. 5—illustrates an energy band diagram for a conventional SONOSstructure, in which stored charge transitions among trapped states (e.g.E_(TA), E_(TD)) to cause leakage.

FIG. 6 is a cross-sectional view of a SONNOS structure.

FIGS. 7-9 illustrate charge retention and migration in a SONNOSstructure such as the one illustrated in FIG. 6.

FIG. 10 illustrates an energy band diagram for a SONNOS structure inwhich stored charge transitions among trapped states (e.g. E_(TA),E_(TD)) to cause leakage.

FIG. 11 is a cross-sectional view of a SONONOS structure.

FIGS. 12-14 illustrate charge retention and migration in a SONONOSstructure such as the one illustrated in FIG. 14.

FIG. 15 illustrates an energy band diagram for a SONONOS structure inwhich stored charge transitions among trapped states (e.g. E_(TA),E_(TD)) to cause leakage.

DETAILED DESCRIPTION

References to “one embodiment” or “an embodiment” do not necessarilyrefer to the same embodiment, although they may.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” Words using the singular or pluralnumber also include the plural or singular number respectively.Additionally, the words “herein,” “above,” “below” and words of similarimport, when used in this application, refer to this application as awhole and not to any particular portions of this application. When theclaims use the word “or” in reference to a list of two or more items,that word covers all of the following interpretations of the word: anyof the items in the list, all of the items in the list and anycombination of the items in the list.

Overview

A charge-storage circuit may be formed with multiple charge storinglayers including multiple nitride layers having differing concentrationsof oxygen, nitrogen and/or silicon. The nitride layers may include atleast a top nitride layer and a bottom nitride layer. At least thebottom nitride layer may comprise silicon oxynitride (e.g.Si_(x)O_(y)N_(x)). The stoichiometric compositions of the layers may betailored or selected such that the lower or bottom nitride has a highoxygen and silicon content, and the top nitride layer has high siliconand a high nitrogen concentration with a low oxygen concentration toproduce a silicon-rich nitride or oxynitride. The silicon-rich andoxygen-rich bottom nitride layer reduces stored charge loss withoutcompromising device speed or an initial (beginning of life) differencebetween program and erase voltages. The silicon-rich, oxygen-lean topnitride layer increases a difference between programming and erasevoltages when the structure is employed in memory devices, therebyimproving device speed, increasing data retention, and extending theoperating life of the device.

However, this structure also has drawbacks in terms of charge retention.Therefore, a middle oxide layer may be formed between the two nitridelayers, forming a split charge-trapping region comprising two nitridelayers separated by a relatively thin oxide layer. In one embodiment,the two nitride layers are approximately equal thicknesses. Each nitridelayer may be at least 30 Å. The middle oxide layer may be at least 5 Å.Some tolerance for process variations is also envisioned, for example+−2 Å. In general, the middle oxide layer will be thin relative to thetwo nitride layers, where ‘thin relative to’ means at least a ratio ofabout 0.75:1. One nitride layer (the bottom layer) may be closer to asubstrate, and oxygen-rich relative to the other (upper) nitride layer.

One process for manufacturing such a semiconductor device includesforming a first oxide layer on a silicon substrate; forming a firstnitride layer on the first oxide layer; applying radical oxidation tothe first nitride layer to form a second oxide layer; and forming asecond nitride layer on the second oxide layer. The first nitride layeris made oxygen-rich relative to the second nitride layer by varying theprocess parameters. For example, each nitride layer may be formed usinga low pressure CVD process using a silicon source, a nitrogen source,and an oxygen-containing gas. With appropriate process parameters, abottom oxynitride layer may be formed that is a silicon-rich andoxygen-rich, and a top nitride layer is may be formed that issilicon-rich, nitrogen-rich, and oxygen-lean. In one embodiment thefirst (lower) nitride layer is formed to a thickness of between 35 Å and80 Å, oxidized to a depth of between 5 Å and 20 Å to form the middleoxide layer, and then the second nitride layer is formed over the middleoxide layer to a thickness of between 30 Å and 60 Å. The first (tunnel)oxide layer on the silicon substrate may be formed to a thickness ofabout 15-20 Å. Again, some tolerance for process variations areenvisioned, for example +−2 Å.

A third oxide layer may be formed over the second nitride layer, to athickness of about 40-50 Å, and a polysilicon or metal gate layer may beformed over the third oxide layer.

Multi-Layer Charge Storing Structure

FIG. 6 is a block diagram illustrating a cross-sectional side view of aportion of a semiconductor memory device 800. The memory device 800includes a SONNOS gate stack 802 including an ONNO structure 804 formedover a surface 106 of silicon layer on a substrate 108. The device 800further includes one or more diffusion regions 110, such as source anddrain regions, aligned to the gate stack 802 and separated by a channelregion 112. Generally, the SONNOS structure 802 includes a gate layer114 formed upon and in contact with the ONNO structure 804. The gate 114is isolated from the substrate 108 by the ONNO structure 804. The ONNOstructure 804 includes a thin, lower oxide layer or tunneling oxidelayer 116 that isolates the gate stack 802 from the channel region 112,a top or blocking oxide layer 120, and a multi-layer charge storinglayer 804 including multiple nitride containing layers. Preferably, themulti-layer charge storing layer 804 includes at least two nitridelayers, including a top nitride layer 818 and a bottom nitride layer819.

FIGS. 7-9 illustrate charge retention and migration in a SONNOSstructure such as the one illustrated in FIG. 6. Charge traps aredistributed through the nitride layers 818, 819, with a distributionthat is uniform under ideal stochiometric conditions (FIG. 7). As aresult of an ERASE (FIG. 8), holes migrate toward the blocking oxide120. Electron charge accumulates at the boundaries of the upper nitridelayer 818 after programming (FIG. 9), and there is less accumulation ofcharge at the lower boundary of the lower nitride layer 819. This mayresult in lower leakage current. Nonetheless, this charge distributionmay lead to charge leakage due to tunneling at the nitride boundaries,as shown for example in FIG. 10, which illustrates how charge maytransition among different trapped states (e.g. E_(TA), E_(TD)) to causeleakage after programming.

Oxide Split Multi-Layer Charge Storing Structure

FIG. 11 is a block diagram illustrating a cross-sectional side view of asemiconductor memory device 1500. The memory device 1500 includes aSONONOS stack 1502 including an ONONO structure 1504 formed over asurface 106 of a substrate 108. Substrate 108 includes one or morediffusion regions 110, such as source and drain regions, aligned to thegate stack 1502 and separated by a channel region 112. Generally, theSONONOS structure 1502 includes a polysilicon or metal gate layer 114formed upon and in contact with the ONONO structure 1504. The gate 114is separated or electrically isolated from the substrate 108 by theONONO structure 1504. The ONONO structure 1504 includes a thin, loweroxide layer or tunneling oxide layer 116 that separates or electricallyisolates the stack 1502 from the channel region 112, a top or blockingoxide layer 120, and a multi-layer charge storing layer 1504 includingmultiple nitride containing layers 1518, 1519. Preferably, themulti-layer charge storing layer 1504 includes at least two nitridelayers, including a top nitride layer 1518, a bottom oxynitride layer1519, and an intermediate oxide layer 1521.

The various layers of the device 1500 may be fabricated to certainthicknesses. Different possibilities for the thicknesses are describedherein, representing possible different embodiments. In general, themiddle oxide layer will be relatively thin in comparison to the twonitride layers. For example, the middle oxide may be betweenapproximately 5 Å and 20 Å. The nitride layers may be the same ordifferent thicknesses as one another, but will typically be at leastapproximately 30 Å. With advances in process technology and materialscience, nitride thicknesses as low as 20 Å may be possible in the nearfuture.

FIGS. 12-14 illustrate charge retention and migration in a SONONOSstructure such as the one illustrated in FIG. 11. Charge traps aredistributed in the two nitride layers 1518, 1519, with a discontinuitywhere the intermediate oxide layer 1521 resides (few or no traps form inthe oxide layer 1521). The majority of traps form in the top nitridelayer 1518. Within each nitride layer, trap distribution is more or lessuniform under ideal stochiometric conditions (FIG. 12). As a result ofan ERASE (FIG. 13), holes migrate toward the blocking oxide 120, but themajority of trapped hole charges form in the top nitride layer 1518.Electron charge accumulates at the boundaries of the upper nitride layer1518 after programming (FIG. 14), and there is less accumulation ofcharge at the lower boundary of the lower nitride layer 1519.Furthermore, due to the intermediate oxide 1521, the probability oftunneling by trapped electron charges in the upper nitride layer 1518 issubstantially reduced. This may result in lower leakage current than forthe structures illustrated in FIG. 1 and FIG. 6. This chargedistribution significantly lowers the probability of tunneling from theupper nitride layer, as shown for example in the energy band diagramsFIG. 15, which illustrate the obstacles to tunneling that chargesencounter as they transition among different trapped states (e.g.E_(TA), E_(TD)) after programming.

Fabrication Techniques

A process of forming a SONOS structure with superior charge retentionbegins with forming a first oxide layer, such as a tunneling oxidelayer, of the ONO structure over a substrate. The substrate may be, forexample, polysilicon, or a silicon surfaced germanium substrate. Next,the first nitride layer of a multi-layer charge storing structure isformed on the first oxide layer. This first or bottom nitride layer maybe formed, for example, by a CVD process including N2O/NH3 and DCS/NH3gas mixtures in ratios and at flow rates tailored to provide asilicon-rich and oxygen-rich oxynitride layer. The first nitride layeris then oxidized to a chosen depth using radical oxidation. This formsthe middle oxide layer. Radical oxidation may be performed, for example,at a temperature of 1000-1100 C using a single wafer tool, or 800-900 Cusing a batch reactor tool. A mixture of H₂ and O₂ gasses may beemployed at a pressure of 300-500 Tor for a batch process, or 10-15 Torusing a single vapor tool, for a time of 1-2 minutes using a singlewafer tool, or 30 min-1 hour using a batch process.

The second nitride layer of the multi-layer charge storing structure isthen formed on the middle oxide layer. The second nitride layer has astoichiometric composition of oxygen, nitrogen and/or silicon differentfrom that of the first (lower) nitride layer. The second nitride layermay be formed or deposited by a CVD process using a process gasincluding DCS/NH3 and N2O/NH3 gas mixtures in ratios and at flow ratestailored to provide a silicon-rich, oxygen-lean top nitride layer.Finally, a second oxide layer of the ONO structure is formed on asurface of the second nitrided layer. This top or blocking oxide layermay be formed or deposited by any suitable means. In one embodiment thetop oxide is a high temperature oxide deposited in a HTO CVD process.Alternatively, the top or blocking oxide layer may be thermally grown,however it will be appreciated that in this embodiment the top nitridethickness may be adjusted or increased as some of the top nitride willbe effectively consumed or oxidized during the process of thermallygrowing the blocking oxide layer. A third option is to oxidize the topnitride layer to a chosen depth using radical oxidation.

In some embodiments, it may be possible perform fabrication by formingthe tunnel oxide layer in one chamber of a CVD tool, then form thebottom oxynitride layer in a second chamber of the CVD tool, thenradical oxidize the lower oxynitride layer in the first chamber, thendeposit more nitride in the second chamber, then radical oxidize thesecond nitride layer in the first chamber again, thus forming thesemiconductor device using a two-chamber process.

Fabrication may further involve forming or depositing a siliconcontaining layer on a surface of the second oxide layer to complete aSONOS stack. The silicon containing layer may, for example, be apolysilicon layer deposited by a CVD process to form a control gate of aSONOS transistor or device. In some embodiments metal may be depositedinstead of polysilicon.

Generally, the substrate 108 may include any known silicon-basedsemiconductor material including silicon, silicon-germanium,silicon-on-insulator, or silicon-on-sapphire substrate. Alternatively,the substrate 108 may include a silicon layer formed on anon-silicon-based semiconductor material, such as gallium-arsenide,germanium, gallium-nitride, or aluminum-phosphide. Preferably, thesubstrate 108 is a doped or undoped silicon substrate.

The lower oxide layer or tunneling oxide layer 116 generally includes arelatively thin layer of silicon dioxide (SiO2) of from about 15 Å toabout 22 Å, and more preferably about 18-20 Å, with some processvariation (e.g. +−1 Å). The tunneling oxide layer 116 may be formed ordeposited by any suitable means including, for example, being thermallygrown or deposited using chemical vapor deposition (CVD). In oneembodiment, the tunnel oxide layer is formed or grown using a steamanneal. This involves a wet-oxidizing process in which the substrate 108is placed in a in a deposition or processing chamber, heated to atemperature from about 700° C. to about 850° C., and exposed to a wetvapor for a predetermined period of time selected based on a desiredthickness of the finished tunneling oxide layer 116. Exemplary processtimes are from about 5 to about 20 minutes. The oxidation may beperformed at atmospheric or at low pressure, or using a dry processunder ambient conditions using either batch or single wafer tools.

The multi-layer charge storing structure generally includes at least twonitride layers having differing compositions of silicon, oxygen andnitrogen, and a middle oxide layer between the two nitride layers. In apreferred embodiment the nitride layers are formed or deposited in a lowpressure CVD process using a silicon source, such as silane (SiH4),chlorosilane (SiH3Cl), dichlorosilane (SiH2Cl2), tetrachlorosilane(SiCl4) or Bis-TertiaryButylAmino Silane (BTBAS), a nitrogen source,such as nitrogen (N2), ammonia (NH3), nitrogen trioxide (NO3) or nitrousoxide (N2O), and an oxygen-containing gas, such as oxygen (O₂) or N2O.Alternatively, gases in which hydrogen has been replaced by deuteriumcan be used, including, for example, the substitution ofdeuterated-ammonia (ND3) for NH3. The substitution of deuterium forhydrogen advantageously passivates Si dangling bonds at thesilicon-oxide interface, thereby increasing the endurance of thedevices.

For example, the lower or bottom oxynitride layer 819, 1519 may bedeposited over the tunneling oxide layer 116 by placing the substrate108 in a deposition chamber and introducing a process gas including N2O,NH3 and DCS, while maintaining the chamber at a pressure of from about 5millitorr (mT) to about 500 mT, and maintaining the substrate at atemperature of from about 700° C. to about 850° C. and more preferablyat least about 780° C., for a period of from about 2.5 minutes to about20 minutes. The process gas may include a first gas mixture of N20 andNH3 mixed in a ratio of from about 8:1 to about 1:8 and a second gasmixture of DCS and NH3 mixed in a ratio of from about 1:7 to about 7:1,and may be introduced at a flow rate of from about 5 to about 200standard cubic centimeters per minute (sccm). A layer produced ordeposited under these condition yields a silicon-rich, oxygen-rich,bottom oxynitride layer 819, that decrease the charge loss rate afterprogramming and after erase, which may be manifested in a small voltageshift in the retention mode.

The top nitride layer 818, 1518 may be deposited in a CVD process usinga process gas including N2O, NH3 and DCS, at a chamber pressure of fromabout 5 mT to about 500 mT, and at a substrate temperature of from about700° C. to about 850° C. and more preferably at least about 780° C., fora period of from about 2.5 minutes to about 20 minutes. The process gasmay include a first gas mixture of N20 and NH3 mixed in a ratio of fromabout 8:1 to about 1:8 and a second gas mixture of DCS and NH3 mixed ina ratio of from about 1:7 to about 7:1, and may be introduced at a flowrate of from about 5 to about 20 sccm. A layer produced or depositedunder these condition yields a silicon-rich, nitrogen-rich, andoxygen-lean top nitride layer 818, 1518.

Preferably, the top nitride layer 818, 1518 is deposited sequentially,after formation of the middle oxide layer, in the same process chamberused to form the bottom oxynitride layer 819, 1519, without altering thetemperature to which the substrate 108 was heated during deposition ofthe bottom oxynitride layer 819, 1519. In one embodiment, the topnitride layer 818, 1518 is deposited sequentially following thedeposition of the bottom oxynitride layer 819, 1519 by (1) moving to adifferent process chamber to form the middle oxide layer by radicaloxidation of the bottom oxynitride layer, (2) returning to the processchamber used to form the bottom oxynitride layer and decreasing the flowrate of the N2O/NH3 gas mixture relative to the DCS/NH3 gas mixture toprovide the desired ratio of the gas mixtures to yield the silicon-rich,nitrogen-rich, and oxygen-lean top nitride layer 818, 1518.

A suitable thickness for the bottom oxynitride layer 819, 1519 may befrom about 30 Å to about 80 Å (with some variance permitted, for example+−10 Å), of which about 5-20 Å may be consumed by radical oxidation toform the middle oxide layer. A suitable thickness for the top nitridelayer 818, 1518 may be at least 30 Å. In certain embodiments, the uppernitride layer may be formed up to 130 Å thick, of which 30-70 Å may beconsumed by radical oxidation to form the top oxide layer. A ratio ofthicknesses between the bottom oxynitride layer and the top nitridelayer is approximately 1:1 in some embodiments, although other ratiosare also possible.

The top oxide layer 120 includes a relatively thick layer of SiO2 offrom about 30 Å to about 70 Å, and more preferably about 40-50 Å. Thetop oxide layer 120 may be formed or deposited by any suitable meansincluding, for example, being thermally grown or deposited using CVD. Inone embodiment, the top oxide layer 120 is a high-temperature-oxide(HTO) deposited using CVD process. This deposition process involvesexposing the substrate 108 to a silicon source, such as silane,chlorosilane, or dichlorosilane, and an oxygen-containing gas, such asO2 or N2O in a deposition chamber at a pressure of from about 50 mT toabout 1000 mT, for a period of from about 10 minutes to about 120minutes while maintaining the substrate at a temperature of from about650° C. to about 850° C.

The top oxide layer 120 may be formed by oxidizing the top nitride layer818, 1518. This may be accomplished in the same chamber used to form thenitride layers 116, 818, 819. The nitride layers 818, 819, 1518, 1519may be formed in a first chamber, and the oxide layers 116, 1521, 120may formed in a second chamber, of a two-chamber tool. Suitable toolsinclude, for example, an ONO AVP, commercially available from AVIZAtechnology of Scotts Valley, Calif.

Although shown and described above as having two nitride layers, i.e., atop and a bottom layer, the present invention is not so limited, and themulti-layer charge storing structure may include a number, n, of nitridelayers, any or all of which may have differing stoichiometriccompositions of oxygen, nitrogen and/or silicon. In particular,multi-layer charge storing structures having up to five, and possiblymore, nitride layers each with differing stoichiometric compositions arecontemplated. At least some of these layers will be separated from theothers by one or more relatively thin oxide layers. However, as will beappreciated by those skilled in the art it is generally desirable toutilize as few layers as possible to accomplish a desired result,reducing the process steps necessary to produce the device, and therebyproviding a simpler and more robust manufacturing process. Moreover,utilizing as few layers as possible also results in higher yields as itis simpler to control the stoichiometric composition and dimensions ofthe fewer layers.

It will further be appreciated that although applicable as part of aSONOS stack in a SONOS memory device, the structure and method of thepresent invention is not so limited, and the ONO structure can be usedin or with any semiconductor technology or in any device requiring acharge storing or dielectric layer or stack including, for example, in asplit gate flash memory, a TaNOS stack, in a 1T (transistor) SONOS cell,a 2T SONOS cell, a 3T SONOS cell, a localized 2-bit cell, and in amultilevel programming or cell, without departing from the scope of theinvention.

Advantages of ONO structures and methods of forming the same accordingto an embodiment of the present invention over previous or conventionalapproaches include: (i) the ability to enhance data retention in memorydevices using the structure by dividing the nitride layer into aplurality of films or layers and tailoring the oxygen, nitrogen andsilicon profile across each layer, with an intermediate oxide layer toreduce the probability of charge tunneling; (ii) the ability to enhancespeed of a memory device without compromising data retention; (iii) theability to meet or exceed data retention and speed specifications formemory devices using an ONO structure of an embodiment of the presentinvention at a temperature of at least about 125° C.; and (iv) provideheavy duty program erase cycles of 100,000 cycles or more.

Implementations and Alternatives

“Logic” refers to signals and/or information that may be applied toinfluence the operation of a device. Software, hardware, and firmwareare examples of logic. Hardware logic may be embodied in circuits. Ingeneral, logic may comprise combinations of software, hardware, and/orfirmware.

Embodiments of the charge retention devices described herein may beemployed in logic circuits to function as machine-memory. Those havingskill in the art will appreciate that there are various logicimplementations that may embody the described structures, and that thepreferred vehicle will vary with the context in which the processes aredeployed. For example, if an implementer determines that speed andaccuracy are paramount, the implementer may opt for a hardware and/orfirmware vehicle; alternatively, if flexibility is paramount, theimplementer may opt for a solely software implementation; or, yet againalternatively, the implementer may opt for some combination of hardware,software, and/or firmware. Hence, there are many vehicles that mayemploy the devices described herein, none of which is inherentlysuperior to the other in that any vehicle to be utilized is a choicedependent upon the context in which the vehicle will be deployed and thespecific concerns (e.g., speed, flexibility, or predictability) of theimplementer, any of which may vary. Those skilled in the art willrecognize that optical aspects of implementations may involveoptically-oriented hardware, software, and or firmware.

The foregoing detailed description has set forth various embodiments ofthe devices and/or processes via the use of block diagrams, flowcharts,and/or examples. Insofar as such block diagrams, flowcharts, and/orexamples contain one or more functions and/or operations, it will beunderstood as notorious by those within the art that each functionand/or operation within such block diagrams, flowcharts, or examples canbe implemented, individually and/or collectively, by a wide range ofhardware, software, firmware, or virtually any combination thereof.

Embodiments of the structures described herein may be employed inApplication Specific Integrated Circuits (ASICs), Field ProgrammableGate Arrays (FPGAs), central processing units (CPUs), digital signalprocessors (DSPs), or other integrated formats. However, those skilledin the art will recognize that some aspects of the embodiments disclosedherein, in whole or in part, can be equivalently implemented indedicated memory circuits, for the purpose of storing digitalinformation for data and/or programs running on one or more computers(e.g., as one or more programs running on one or more computer systems),as one or more programs running on one or more processors (e.g., as oneor more programs running on one or more microprocessors), as firmware,or as virtually any combination thereof.

In a general sense, those skilled in the art will recognize that thevarious structures described herein may be embodied, individually and/orcollectively, by a wide range of electrical circuitry. As used herein“electrical circuitry” includes, but is not limited to, electricalcircuitry having at least one discrete electrical circuit, electricalcircuitry having at least one integrated circuit, electrical circuitryhaving at least one application specific integrated circuit, electricalcircuitry forming a general purpose computing device configured by acomputer program (e.g., a general purpose computer configured by acomputer program which at least partially carries out processes and/ordevices described herein, or a microprocessor configured by a computerprogram which at least partially carries out processes and/or devicesdescribed herein), electrical circuitry forming a memory device (e.g.,forms of random access memory), and/or electrical circuitry forming acommunications device (e.g., a modem, communications switch, oroptical-electrical equipment).

Those skilled in the art will recognize that it is common within the artto describe devices and/or processes in the fashion set forth herein,and thereafter use standard engineering practices to integrate suchdescribed devices and/or processes into larger systems. That is, atleast a portion of the devices and/or processes described herein can beintegrated into a network processing system without an undue amount ofexperimentation.

1. A memory device comprising: a split charge-trapping region comprisingat least two nitride layers, the at least two nitride layers separatedby one or more relatively thin oxide layers, wherein the at least twonitride layers comprise one nitride layer closer to a substrate that isoxygen rich relative to another nitride layer on the other side of theone or more relatively thin oxide layers, and wherein charge traps aredistributed in the at least two nitride layers, and the nitride layer onthe other side of the one or more relatively thin oxide layers comprisesa majority of the charge traps.
 2. The memory device of claim 1, whereinthe charge-trapping region further comprises: two nitride layers each atleast 30 Å thick.
 3. The memory device of claim 1, wherein the at leastone relatively thin oxide layer further comprises: a single oxide layerbetween two nitride layers, the oxide layer being at least 5 Å thick. 4.The memory device of claim 1, wherein the charge-trapping region furthercomprises: two nitride layers of approximately equal thickness.
 5. Thememory device of claim 1, wherein the one nitride layer closer to thesubstrate comprises an oxygen rich oxynitride.
 6. The memory device ofclaim 5, wherein the nitride layer on the other side of the one or morerelatively thin oxide layers comprises an oxynitride having a highsilicon and a high nitrogen concentration to produce an oxygen-pooroxynitride layer.
 7. A semiconductor device comprising: a substrate; afirst oxide layer formed on the substrate; and a charge-trapping regioncomprising: an oxygen-rich nitride layer formed on the first oxidelayer; a second oxide layer formed on the oxygen-rich nitride layer; andan oxygen-poor nitride layer formed on the second oxide layer.
 8. Thesemiconductor device of claim 7, further comprising: a third oxide layerformed on the oxygen-poor nitride layer.
 9. The semiconductor device ofclaim 8, further comprising: a polysilicon or metal layer formed on thethird oxide layer.
 10. The semiconductor device of claim 7, whereincharge traps are distributed in the oxygen-rich nitride layer and theoxygen-poor nitride layer, and wherein the oxygen-poor nitride layercomprises a majority of the charge traps.
 11. The semiconductor deviceof claim 7, wherein the oxygen-rich nitride layer and the oxygen-poornitride layer comprise approximately equal thicknesses of greater than30 Å.
 12. The semiconductor device of claim 7, wherein the oxygen-richnitride layer comprises an oxygen-rich oxynitride layer.
 13. Asemiconductor device comprising: a first oxide layer on a substrate; anda charge-trapping region formed on the first oxide layer comprising: afirst nitride layer formed on the first oxide layer and a second nitridelayer of approximately equal thicknesses and with charge trapsdistributed therein, wherein a majority of the charge traps aredistributed in the second nitride layer; a second oxide layer formed onthe first nitride layer and separating the first nitride layer from thesecond nitride layer, and wherein the first nitride layer and the secondnitride layer comprise oxynitride and the first nitride layer is oxygenrich relative to the second nitride layer.
 14. The semiconductor deviceof claim 13, wherein the second nitride layer comprises an oxynitridehaving a high silicon and a high nitrogen concentration to produce anoxynitride layer that is oxygen poor relative to the first nitridelayer.
 15. A process of manufacturing a semiconductor device, theprocess comprising: forming a first oxide layer on a substrate; forminga first nitride layer on the first oxide layer; applying radicaloxidation to the first nitride layer to form a second oxide layer; andforming a second nitride layer on the second oxide layer, wherein thefirst nitride layer is oxygen rich relative to the second nitride layer,and wherein charge traps are distributed in the first nitride layer andthe second nitride layer, and the second nitride layer comprises amajority of the charge traps.
 16. The process of manufacturing asemiconductor device of claim 15, wherein forming the first nitridelayer on the first oxide layer further comprises: forming the firstnitride layer oxygen-rich relative to the second nitride layer.
 17. Theprocess of manufacturing a semiconductor device of claim 15, whereinforming the first nitride layer on the first oxide layer furthercomprises: forming the first nitride layer to a thickness of at least 30Å.
 18. The process of manufacturing a semiconductor device of claim 15,wherein applying radical oxidation to the first nitride layer to formthe second oxide layer further comprises: oxidizing the first nitridelayer to a depth of at least 5 Å.
 19. The process of manufacturing asemiconductor device of claim 15, wherein forming a second nitride layeron the second oxide layer further comprises: forming the second nitridelayer to a thickness of at least 30 Å.
 20. The process of manufacturinga semiconductor device of claim 15, wherein forming the first oxidelayer on the substrate further comprises: forming the first oxide layerto a thickness of at least 15 Å.
 21. The process of manufacturing asemiconductor device of claim 15, further comprising: forming a thirdoxide layer over the second nitride layer.
 22. The process ofmanufacturing a semiconductor device of claim 21, wherein forming thethird oxide layer over the second nitride layer further comprises:forming the third oxide layer to a thickness of at least 30 Å.
 23. Theprocess of manufacturing a semiconductor device of claim 15, furthercomprising: forming the nitride layers in a first process chamber, andthe oxide layers in a second process chamber, by alternately moving awafer back and forth between the two chambers.
 24. The process ofmanufacturing a semiconductor device of claim 21, further comprising:each oxide layer formed using radical oxidation of an underlying layer.